In an IP Multimedia Subsystem (IMS) network, when the User Equipment (UE) or a Public Service Identifier (PSI) of an IMS user acts as a called, an Interrogating-Call Session Control Function (I-CSCF) module in the called network inquires a Home Subscriber Server (HSS) about the registration location of the IMS user through a command Cx-Location-Query.
If the IMS user subscribes to an unregistered service (e.g. transfer-to-voice-mailbox), the HSS returns a Serving-CSCF (S-CSCF) for processing the registered service of the IMS user to the I-CSCF through a command Cx-Location-QueryResp when the IMS user is in a REGISTERED state, the I-CSCF sends a Session Initiation Protocol (SIP) request to the S-CSCF returned from the HSS, and the S-CSCF processes the registered service of the IMS user.
If the IMS user subscribes to an unregistered service (e.g. transfer-to-voice-mailbox), the HSS returns an S-CSCF for processing the unregistered service of the IMS user to the I-CSCF through a command Cx-Location-QueryResp when the IMS user is in an UNREGISTERED state or a relevant identity identifier (ID) (e.g. the user equipment with the same number as the IMS user) has been registered or the relevant ID (e.g. the user equipment with the same number as the IMS user) is in an UNREGISTERED state, the I-CSCF sends a SIP request to the S-CSCF returned from the HSS, and the S-CSCF processes the unregistered service of the IMS user.
If the IMS user subscribes to an unregistered service (e.g. transfer-to-voice-mailbox), the HSS returns a set of S-CSCF capabilities required by the IMS user to the I-CSCF through a command Cx-Location-QueryResp when the IMS user is in a NOT REGISTERED state and the relevant ID (e.g. the user equipment with the same number as the IMS user) is also in a NOT REGISTERED state, the I-CSCF selects an satisfactory S-CSCF in accordance with the set of S-CSCF capabilities required by the IMS user and sends a SIP request to the selected S-CSCF, and the S-CSCF processes the unregistered service of the IMS user.
In the prior art, because the I-CSCF may be stateless, i.e. the I-CSCF, when sending a SIP message, may not await a response to the message, it may result in the following disadvantages.
When the S-CSCF forwards the SIP request or processes the registered service or unregistered service of the IMS user, the session will time out if the S-CSCF does not respond due to a device failure, or communication interruption, etc. so that the SIP request from the calling side will fail due to the time out (which may result in call loss).
Further, when the IMS user is in a REGISTERED state, the registered service of the called IMS user can not be executed, and even a network prompt like “The subscriber you dialed is out of service, please redial later” present in the existing network can not be provided.
When the called IMS user is in an UNREGISTERED state or the relevant ID has been registered or the relevant ID is in an UNREGISTERED state, the unregistered service of the called IMS user can not be executed, so that the IMS user can not be provided with a proper service for a long time over the IMS network (the IMS user can not be provided with a proper service over the IMS network until the S-CSCF resumes the service or the called IMS user registers successfully with another normal S-CSCF after powering on, which may last for a long time such as hours, days or even longer).
When the IMS user is in a NOT REGISTERED state and the relevant ID is also in a NOT REGISTERED state, the unregistered service of the called IMS user can not be executed.